This invention relates to a process for preparing poly(alkylene carbonates).
Poly(alkylene carbonates) (sometimes referred to herein as PACs or PAC polyols) are copolymers prepared by reacting carbon dioxide or an alkylene carbonate with an alkylene oxide to form a polymer containing repeating alkylene carbonate units. PACs are useful, for example, as raw materials for the production of polyurethanes.
PACs are conventionally prepared by reacting carbon dioxide or an alkylene carbonate with an alkylene oxide such as ethylene oxide or propylene oxide at elevated pressure and temperature. Two major classes of catalysts are used for this reaction. Alkali metal salts such as potassium carbonate are used to prepare a PAC having a carbon dioxide content (the weight percentage of the polymer attributable to C(O)O groups) of about 0.1 to about 15%. Stannate salts such as sodium stannate provide a PAC having a carbon dioxide content of about 25-33%. When these catalysts are used, the carbon dioxide content of the polymer is controlled almost entirely by the choice of catalyst, and variations in the ratios of materials and in the reaction conditions have little effect on the composition of the product polymer. In addition, these catalysts require that the polymerization be conducted very slowly, or under very high pressures, or both. Another disadvantage is that the product PACs usually contain high levels of by-products notably alkylene carbonates.
Unfortunately, PACs having carbon dioxide contents from about 10-25% are of particular interest in preparing polyurethanes. PACs having such carbon dioxide contents display the best combination of solvent resistance and low viscosity. These materials cannot be prepared in any quantity directly for ethylene oxide and carbon dioxide using the aforementioned alkali metal salts and stannate catalysts. PACs having such carbon dioxide contents can be prepared using certain catalysts such as arsenate salts. However, these catalysts are very toxic, and the rate of polymerization achieved with these catalysts is too slow to be commercially viable.
Accordingly, it would be desirable to provide a process whereby a PAC can be prepared having a carbon dioxide content which can be varied as desired. It would also be desirable to provide a process which provides a higher reaction rate, and, preferably, lower reaction pressures than conventional processes.